This invention relates to spatial filters and more particularly to a spatial filter having an aerodynamic window for use with high power lasers.
Laser systems capable of providing an output beam having high optical quality effectively discriminate against nonuniformities within the beam such as high order transverse modes and high frequency phase distortions. One method well known in the art of discriminating against nonuniformities within a beam is to provide spatial filtering either of the radiation within the resonator of a laser or of the laser beam after it passes out of the resonator to filter out undesirable elements. For example, a spatial filter located at a common focal plane of a negative branch confocal optical system forming part of a resonator of a ring laser provides filtering of unwanted high order modes and high frequency phase distortions of radiation circulating within the resonator and results in an improved cross-sectional power density distribution in an output beam. The optical system typically includes a spherical mirror to focus radiation circulating within the resonator to a diffraction limited spot at the common focal plane typically at the spatial filter and a second spherical mirror to collect radiation expanding from the focal plane to provide a collimated beam of radiation to the active gain region of the resonator.
Spatial filtering is effective to discriminate against nonuniformities when the power density at the focal point is sufficiently low to avoid thermal blooming and/or breakdown of the gas or air in the transmission medium. Gas breakdown and/or thermal blooming results, however, when a high power laser beam is focused to a diffraction limited spot. An additional adverse effect is introduced when high power laser radiation is incident upon a metallic or conducting surface of a spatial filter. The interaction results in the formation of a plasma adjacent the filter which will attenuate or interfere with the beam transmission.
One method of avoiding the gas breakdown and/or thermal blooming problem is to spatial filter the beam in a vacuum or low pressure environment. This typically requires enclosing the filter in a housing having windows made of material transparent at the wavelength of the laser radiation. The radiation is focused through one window to a diffraction limited spot at the spatial filter. The radiation then expands from the spot through the filter and passes out of the environment through the second window. However, absorption of high power radiation within the windows can result in damage to windows and loss of power in the beam. Also, reflections at the window surfaces further increases the loss of power in the beam and may result in damage to other components of the laser system.